Process and apparatus for surface hardening hardenable steels

ABSTRACT

This invention relates to surface hardening hardenable steels with the plasma flame from a nozzleless plasma torch. The torch has an inner rod electrode and an outer electrode, which extends axially further than the rod electrode. A stream of ionizable gas passes through the torch and an electric arc discharge is sustained between the electrodes to create a plasma flame. The steel to be hardened is subjected to the flame to heat its surface at an extremely high rate to form metastable austenite. Then further energy is introduced, such as by mechanical shock, to transform the austenite into fine-grained martensite.

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[72] Inventors gal'lllsvviobodal7 .v "I 5 fl f gg (jif d ec ar gasseienna Maximilian Pater, Albert Bohlergasse 9, UNYTED, STATES PATENTSKapfenberg Styria, boh of Austria 2,527,287 10/1950 Ziegler et al 148/ l2.3 [2]] Appl. No. 700,975 2,717,846 9/1955 148/124 [22] Filed Jan. 26,1968 2,922,869 l/l960 219/75 45 Patented 0a. 26, 1971 3,146,336 8/1964219/ 121 [32] Priority Sept. 23, 1964 3,240,639 3/1966 148/154 X [33]Austria Primary Examiner-Charles N. Lovell [31] 8121/64 Attorney-Holman,Holman & Stern Continuation-impart of application Ser. No. 486,053,Sept. 9, 1965, now abandoned.

ABSTRACT: This invention relates to surface hardening hardenable steelswith the plasma flame from a nozzleless plasma torch. The torch has aninner rod electrode and an outer electrode, which extends axiallyfurther than the rod [54] PROCESS AND APPARATUS FOR SURFACE HARDENINGHARDENABLE STEELS 4 Chums 1 Drawing electrode. A stream of ionizable gaspasses through the torch [52] U.S.Cl 148/143, and an electric arcdischarge is sustained between the elecl48/4, l48/l2.4, 148/39, 219/121P trodes to create a plasma flame. The steel to be hardened is [51] Int.Cl C2ld l/06 subjected to the flame to heat its surface at an extremelyhigh [50] Field of Search 148/12, rate to form metastable austenite.Then further energy is inl2.4, 12.9, 4,31,39, 134, 143, l54;2l9/75, 121;troduced, such as by mechanical shock, to transform the 6/5 austeniteinto fine-grained martensite.

PROCESS AND APPARATUS FOR SURFACE HARDENING HARDENABLE STEELS Thisapplication is a continuation-in-part application of Ser. No. 486,053,filed Sept. 9, 1965 now abandoned.

According to current technology hardenable steels can be surfacehardened in such a manner that an austenitic layer, which is metastableat room temperature, is formed in the first stage and this layer istransformed in the second stage by an addition of further energy, e.g.by a mechanical shock, into a fine-grained martensitic structure, whichis very hard and very tough. The formation of the metastable austeniticsurface layer, which austenite represents a kind of undercooledstructure on a steel which is normally ferritic at room temperature,requires an extremely fast heating thereof into the temperature regionbetween the upper transformation point and the melting point of thematerial. The rapid dissipation of heat which is required is mainlyeffected by the workpiece itself, and for this reason, does not usuallycall for additional steps. The transformation into martensite usuallyoccurs when the austenitic surface of the workpiece has to withstand anykind of mechanical strain or shock during use.

This surface-hardening process carried out in two stages eliminates anumber of disadvantages which are involved in the usualsurface-hardening process. More particularly, the two-stage process canbe carried out so as to avoid heat treatment cracks or the formation ofa heat valley or zone of reduced hardness in heat-treated and thensurface-hardened steel members.

The rapid heating of the surface may be effected, e.g., by means of asteel disc which is rotated at high speed and has a smooth end face,against which the workpiece is forced. In principle, this heating couldbe effected by all means which enable high energy concentrations to beachieved, such as electron beams, laser beams or plasma beams.

A disadvantage of these means for supplying energy so as to effect anextremely rapid heating of the surface resides in that only a relativelysmall part of the surface can be heated at a time so that the hardeningtreatment of large surfaces, or of intricately shaped parts must becarried out in numerous steps, preferably in continuous succession. Itis very difficult, however, to accomplish a uniform surface hardening oflarge surfaces in this way. Besides, the use of energy sources having anextremely high energy concentration requires the provision of expensiveequipment, which prevents large-scale applications of the two-stagesurface-hardening process.

For this reason there is a desire for an inexpensive energy source whichenables a sufficiently high energy concentration. Also, such energysource should be conformable as closely as possible to the shape of thesurface to be treated so that the number of partial treatments which arerequired is minimized and are ideally reduced to a single treatment.

Such energy source has been found in the form of a novel plasma torch.This torch comprises an additional source of charge carriers, ratherthan a nozzle, for shaping the plasma flame, and this charge carriersource introduces charge carriers into the plasma so as to eliminate thequasi-neutral state thereof. The shaping or focusing of the plasma beamor plasma flame may then be effected by a variation of the threecontrolling variables a. current value b. gas feed rate c. nature offeed gas. By the term nature of feed gas" is for instance meant whetherthe gas used is pure argon, or a mixture of argon and nitrogen.Different gases dictate different cross sections of the plasma beam.

Such a plasma torch is extremely simple in structure and for this reasoncan be manufactured at low cost. It can be operated with direct currentor alternating current. The use of alternating current affords theadditional advantage in that the rectifier means required for operationwith direct current eliminated so that the cost of the equipment isfurther reduced. It may also be mentioned that the expensive nozzlesrequired for the previous plasma torches have a relatively short life sothat the elimination of such nozzles results also in a substantialreduction of the cost of operating such equipment. in this specificationand the appended claims, the term plasma is not restricted to gaseswhich are in a quasi-neutral state.

An illustrative apparatus for carrying out the process according to theinvention is shown on the accompanying drawing and comprises acylindrical torch tube 3, which is closed at one end by an insulator 2and which contains a generally axially extending electrode rod 1 ofthoriated tungsten. At its other end, the tube 3 may be formed with acylindrical recess, which extends around the entire inside periphery ofthe tube and contains a tubular electrode 5, which consists also ofthoriated tungsten. The inside surface of the electrode 5 is flush withthe inside wall of the torch tube. A gas supply pipe 4 is laterallyattached to the torch tube 3. This pipe may tangentially or radiallyjoin the tube 3.

If the torch tube 3 consists of a conducting material, such as steel,one of the current supply conductors 6 is connected to the tube 3 whichrepresents the outer electrode and the other to the inner electrode 1.

An electric arc of DC or AC is maintained between the rodshapedelectrode 1 and the tubular electrode 5. The lines of the electric fieldof said are are inclined to the direction of flow of the gas which formsthe plasma. A magnetic field is thus established, which has apredominant azimuthal component and causes a radial separation betweenthe positive and negative charge carriers. The positive ions migrateinwardly and the electrons migrate outwardly. A positive acceleratingcharge is thus established before the rod-shaped electrode 1 so that thelatter is caused to emit additional electrons and constitutes anadditional electron source, which eliminates the quasi-neutral state ofthe plasma. The annular electrode 5 is another electron source and isnot water-cooled. When heated by the electric arc, the electrode 5 alsoemits electrons. Electron densities of an order of IO" electrons percubic centimeter have been measured in the emerging plasma. Theadditional electrons emitted by the rod-shaped electrode 1 stabilize thecross section of the plasma, which is focused by the azimuthal magneticfield of the electric arc. The ion cloud before the rod-shaped electrode1 would otherwise collapse under the action of the repelling forces ofthe positive charge carriers.

EXAMPLE 1 An inner electrode 6 millimeters in diameter was insulatedlymounted in a steel tube which was l2 millimeters in inside diameter. Anannular recess in the inside surface of the steel tube contained anannular insert, which has a length of l5 millimeters and consisted of asintered mixture of tungsten carbide and cobalt. The inside surface ofthe liner was flush with the unrecessed part of the inside surface ofthe steel tube. The inner electrode extended to one half of the lengthof this insert, which constituted an outer electrode. An AC voltage wasapplied across the two electrodes so that an electric arc was formedhaving an arc voltage of 2025 volts and an are current of 200 amperes.Argon at a rate of 5 standard liters per minute was used as aplasma-forming gas. These operating conditions resulted in a plasma jetabout 6 millimeters in diameter, which emerged from the tube.

The feeding speed of the torch or of the steel member to be hardened was200-300 millimeters per minute. The distance from the outlet opening ofthe torch to the surface to be hardened was 5-10 millimeters. Withinthis range, there was equalizing short circuit currents between theradially separated zones occupied by the positive and negative chargecarriers in the plasma jet. Incremental elements of the surface of theworkpiece were thus heated by the surface current flowing in suchelements to the temperature required for austenitizing. At a greaterdistance, radially separate zones having charges of opposite polaritieswere no longer observed. Cooling was effected by the quenching action ofthe mass of steel which adjoined the heated zone and had remained cold.

EXAMPLE 2 The same operating conditions were adopted as in example 1,except that the gas rate was increased to standard liters per minute.The diameter of the plasma jet was thus reduced to 4 millimeters.

The same operating conditions were adopted as in example 1, except thatthe arc current was increased to 300 amperes. The diameter of the plasmajet was thus increased to 3 millimeters.

EXAMPLE 4 The same operating conditions were adopted as in example 1,except that a gas mixture of 50 percent argon and 50 percent nitrogenwas used. The diameter of the plasma jet was thus reduced to 1.5-2millimeters.

The invention provides a process of surface-hardening hardenable steelsand resides in that in the known two-stage surface-hardening process atleast the extremely rapid heating required in the first stage iseffected with a nozzleless plasma torch.

The process may comprise passing a stream of ionizable gas through anannular passage and afterwards through a tubular passage, axiallyimmediately succeeding said annular passage, sustaining an electric DCor AC arc discharge between the radially inside boundary surface of saidannular passage at said outlet and thereof and the radially insideboundary surface of said tubular passage. The azimuthal magnetic fieldof said discharge attracts carriers of a charge having a first signtowards the axis of said annular passage at the outlet end thereof andcarriers of a charge having a second sign which is opposite to saidfirst sign radially outwardly in a region which is axially beyond theoutlet of said annular passage. The steel surface to be hardened isbrought in the zone in which the charge carriers of opposite signs areradially separated. It is heated by surface streams flowing from onezone to the other.

The advantage which can be achieved in this process resides mainly inthat the two-stage surface hardening process can be carried out with theaid of an energy source which is very simple in structure and for thisreason inexpensive, and that the elimination of the nozzles which arerequired in the usual plasma torch and subject to a rapid wear, resultsin an avoidance of trouble in operation and in a reduction of theoperating costs.

The use of a nozzleless plasma torch which is recommended according tothe invention enables also a considerable adustment of the form of theplasma beam and of the rate at which energy is supplied in view of theshape of the workpiece surface to be hardened. Furthermore, incontinuous surfacehardening treatments, the shape of the plasma beam andthe rate of energy supply can be varied in accordance to the form of thesurface even during the treatment.

For instance, surface hardening the working parts of tools, particularlythe cutting edges of tools, can be effected in a very simple manner bymeans of this process. The process according to the invention may alsobe used to advantage for hardening slideways, dovetail surfaces, or thelike. Cylindrical parts are suitably rotated and are treated in thisstate with a plasma flame which has a low energy concentration and isout of focus as far as possible.

Nozzleless plasma torches operated with alternating current can becontrolled in the same way as plasma torches for direct currentoperation, and have the additional advantage that the equipment is evensimpler in structure because the rectifier plant required for directcurrent operation is eliminated. It has been observed, that the radialseparation of charge carriers beyond the outlet end of the annularpassage was partly sustained even if alternating current was used, whichfact is in accordance with the plasma theory.

The present invention eliminates a major difficulty which previouslyobstructed the large-scale use of the two-stage surface-hardeningprocess. What is claimed is:

l. A method of surface hardening hardenable steels which comprises thesteps of passing a stream of ionizable gas through an electric arc of anozzleless plasma torch, which arc is sustained between the tip of a rodelectrode and a surrounding tubular electrode, the lines of the electricfield of said arc being inclined to the direction of flow of said gas,said gas being ionized in said are and the azimuthal magnetic field ofsaid are attracting carriers of a charge having a first sign radiallyinwardly towards the axis of said tubular electrode and attractingcarriers of a charge having a second sign, opposite to said first sign,radially outwardly away from said axis; bringing the surface of aworkpiece to be hardened into the zone in which the charge carriers ofopposite signs are radially separated, whereby the surface layer israpidly heated by recombination energy resulting when the opposite signcharge carriers recombine, said surface layer being raised to atemperature between the upper transformation point and the melting pointof said steel to form after rapid dissipation of heat by the workpiecean austenitic layer which is metastable at room temperature andimparting an effective amount of energy to said metastable layer totransform same into finegrained martensite, whereby the surface ishardened.

2. The method as claimed in claim 1, in which said nozzle less plasmatorch has an additional source of charge carriers, said charge carriersource introducing charge carriers into the plasma so as to eliminatethe quasi-neutral state thereof.

3. The method as claimed in claim 2, wherein the form of the plasmaflame and the rate at which energy is supplied by said flame to saidsurface in accordance with the configuration of the surface to behardened is controlled by selecting a combination of the following threevariables:

a. current value b. gas feed rate c. nature of feed gas.

4. A process as set forth in claim 1, in which said torch is operatedwith alternating current.

2. The method as claimed in claim 1, in which said nozzleless plasmatorch has an additional source of charge carriers, said charge carriersource introducing charge carriers into the plasma so as to eliminatethe quasi-neutral state thereof.
 3. The method as claimed in claim 2wherein the form of the plasma flame and the rate at which energy issupplied by said flame to said surface in accordance with theconfiguration of the surface to be hardened is controlled by selecting acombination of the following three variables: a. current value b. gasfeed rate c. nature of feed gas.
 4. A process as set forth in claim 1 inwhich said torch is operated with alternating current.